[038] Forward Converter Design - Part V Snubber and Clamp Circuits

Snubber and clamp design for a forward converter.

Introduction

This article continues the series in which Dr. Ridley documents the processes involved in taking a power supply from the initial design to the full-power prototype. In part V, attention is turned to the secondary side of the converter, where snubbers and clamps are added to protect the output rectifiers.

Secondary Catch Diode Ringing Voltage

In Part IV of this series of articles [1], the primary voltage spikes on the switching FETs were properly controlled with improved layout and high-frequency bypass capacitors. This greatly improved the ruggedness of the converter.

Voltage spikes and ringing can also be seen on the secondary of the converter. Figure 1 shows the ringing across the catch diode with 100 VAC applied to the primary of the converter (maximum voltage rating is 280 VAC). This ringing must be suppressed before higher voltages can be applied to the converter. A peak voltage of 175 V is seen at the leading edge of the waveform, more than three times higher than the anticipated square wave voltage of 50 V. If this is not controlled, the secondary rectifiers will fail when full voltage is applied.